This invention relates to a composition for gas treatment to remove heavy metals, particularly mercury, from gas streams, particularly flue gas streams, and processes and systems for making and using the composition. In particular, the invention relates to a sorbent for removal of mercury from flue gas and processes and systems for making and using the sorbent.
In August 2000, the National Research Council completed a study that determined that the U.S. Environmental Protection Agency's (EPA) conservative exposure reference dose of 0.0001 mg mercury/kg body weight/day was scientifically justified to protect against harmful neurological effects during fetal development and early childhood. Subsequently, in December 2000, EPA announced its intention to regulate mercury and other air toxics emissions from coal- and oil-fired power plants. The pending regulation has created an impetus in the utility industry to find cost-effective solutions to meet the impending mercury emission standards.
Domestic coal-fired power plants emit a total of about fifty metric tons of mercury into the atmosphere annually—approximately one third of all anthropogenic mercury emissions in the U.S. A coal-fired utility boiler emits several mercury species, predominantly in the vapor-phase in boiler flue gas, including elemental mercury, and ionic mercury in mercuric chloride (HgCl.sub.2) and mercuric oxide (HgO)—in different proportions, depending on the characteristics of the coal being burned and on the combustion conditions.
Today, municipal solid waste (MSW) incinerators and medical waste combustors predominantly utilize the best commercially available control technology for reduction of mercury emissions: adsorption of mercury species onto activated carbon. Although fairly effective for MSW incinerators, activated carbon is a less appealing solution for coal-fired flue gas streams because of the dramatic difference in mercury concentrations. Regulations for mercury control from municipal and medical waste incinerators specify outlet emission levels of no more than fifty micrograms per cubic meter. In coal-fired flue gas streams, typical uncontrolled mercury concentrations are on the order of ten micrograms per cubic meter. Thus, reduction of mercury emissions from coal combustion flue gases presents a unique challenge in that the mercury is present in low concentrations in very large volumes of flue gas.
Fixed beds of zeolites and carbons have been proposed for a variety of mercury-control applications, but none has been developed specifically for control of mercury in coal combustion flue gas. Products in this class include Lurgi GmbH's (Frankfurt, Germany) Medisorbon and Calgon Carbon Corporation's (Pittsburgh, Pa.) HGR.
Calcium carbonate (limestone), calcium oxide (lime), and calcium hydroxide (slaked lime) are employed in flue gas desulfurization (FGD). Sulfur dioxide in flue gas reacts with these materials to yield solid calcium sulfite. It is known that some of the mercury in the flue gas is removed in the flue gas desulfurization processes employed by electric utilities, however the proportion of mercury removed falls short of the goals set by EPA. Some installed FGD systems allow relatively pure calcium sulfite to be oxidized to calcium sulfate (FGD Gypsum) which may be sold for use in wallboard. Unlike FGD gypsum, which can be sold, most power plants have to pay to dispose of sulfite-rich scrubber material. Out of 18 million tons of sulfite-rich scrubber material produced by coal-burning power plants in 2000, 3 million tons were disposed of as wet by-product, 12 million tons were disposed of in landfills as dry by-product, and only 1 million tons were used for any meaningful purpose at coal-burning electric utility sites. This material presents environmental challenges due to concerns associated with long-term impacts of calcium-sulfite landfills. A beneficial use for FGD calcium sulfite-rich by-product, which is often admixed with varying amounts of unreacted calcium carbonate, oxide, or hydroxide, as well as coal combustion ash, is being sought by coal-burning electric utilities.
At present, the injection of activated carbon is generally considered to be the best available demonstrated control technology for reduction of mercury emissions from coal-fired power plants that do not have wet scrubbers (about seventy-five percent of all such plants in the U.S.). Tests of carbon injection, both activated and chemically impregnated, have been reported in the technical literature. In order to achieve EPA's goal of removing 90% of the low mercury concentrations found in coal combustion flue gases, projected injection rates for activated carbon are on the order of 10,000 to more than 20,000 pounds of activated carbon for each pound of mercury removed, depending on the physical characteristics of the activated carbon, and the concentration and speciation of mercury in the flue-gas. The cost to implement effective activated carbon mercury control systems has been estimated by the U.S. Department of Energy (DOE) to be on the order of US$60,000 per pound of mercury removed.
Activated carbon injection rates for effective mercury control at different facilities have been found to be widely variable and are explained by the dependence of the sorption process on flue gas temperature and composition, efficiency of dispersion of the activated carbon throughout the flue gas stream, mercury speciation and also on fly ash chemistry. When employed for mercury control, some of the carbon becomes part of the ash collected by particulate-control devices and would be expected to make the fly ash unsuitable for incorporation into concrete. This impact on the marketability of collected fly ash can substantially increase the effective cost of mercury control for a coal-fired power plant, and more of this major coal combustion by-product would become a waste to occupy landfill space.
In addition to the economic drawbacks presented by the use of activated carbon sorbent for mercury control, technical viability issues remain to be resolved. Coal-fired combustion flue gas streams include trace amounts of acid gases, including SO.sub.2, NO and NO.sub.2, and HCl. This mix of acid gases has been shown to degrade the performance of some of the chemically treated activated carbons and other sorbents such as noble-metal-impregnated alumina.
Regenerable sorbents with an initial cost roughly equivalent to activated carbon have been developed with the aim of reducing the overall cost of mercury removal through recycle of the sorbent. These sorbents employ a phyllosilicate mineral substrate and precipitate a polyvalent sulfide from aqueous solution onto the mineral's surface in a multistep aqueous process (U.S. Pat. No. 6,719,828 to Lovell et al.). Collecting and processing such a sorbent to regenerate such a fine particulate material would be expected to present significant unresolved challenges for the typical coal-fired power plant.
While micro-porosity is a critical characteristic of an efficient sorbent for mercury from flue gases, mass transfer of gaseous mercury by diffusion from the bulk flue gas to the solid surface can limit capture of mercury; diffusion within a porous sorbent is not believed to be rate-limiting (Status review of mercury control options for coal-fired power plants; John H. Pavlish, et al.; Energy and Environmental Research Center; 2003). Reducing the size of the sorbent particles and increasing their dispersion in the gas stream enhances control, but large quantities of sorbent are required in all instances. Pavlish et al. found that to achieve 90% mercury removal in 2 seconds residence time by activated carbon injection required a minimum carbon-to-mercury mass ratio of about 3,000:1 for 4 micron particles and about 18,000:1 for 10 micron particles. Assuming constant density for the carbon particles and more spherical particles as the particle size decreases, the data of Pavlish et al. indicate that approximately the same number of 10 micron particles or 4 micron particles are required to achieve the 90% mercury removal. Chemical treatments to enhance the ability of activated carbon and micro-porous mineral substrates to adsorb and fix mercury increase the cost per pound of sorbent, thus substantially increasing the cost of overcoming this mass transfer limitation. An effective sorbent with a cost far below the cost for activated carbon is needed to allow the necessary large number of particulates to be dispersed within the flue gas stream to cost-effectively overcome this mass transfer by diffusion limitation.
Thus a pressing need exists for a mercury sorbent which is capable of being dispersed in a coal combustion flue gas stream as very small particulates, is capable of adsorbing both elemental and ionic mercury species, is substantially less expensive than activated carbon, and has characteristics which allow it to be incorporated into concrete along with coal combustion ash. A preferred embodiment of the present invention utilizes calcium sulfite-rich FGD by-product material for the production of an effective low-cost calcium sulfide-rich mercury sorbent.
U.S. Pat. No. 4,193,811 to Ferm teaches that alkaline earth metal polysulfides, particularly calcium polysulfide, are beneficial additives to concrete in that they act as strength enhancers.
U.S. Pat. No. 3,194,629 to Dreibelbis et al. discloses impregnation of activated carbon with elemental sulfur as a sorbent for removing mercury from gases.
U.S. Pat. No. 3,873,581 to Fitzpatrick et al. discloses a process for reducing the level of contaminating mercury in aqueous solutions. The process is applied to aqueous solutions and not to gases and it relies on treating an adsorbent with a mercury-reactive factor. Disclosed absorbents are titania, alumina, silica, ferric oxide, stannic oxide, magnesium oxide, kaolin, carbon, calcium sulfate, activated charcoal, activated carbon, activated alumina, activated clay or diatomaceous earth.
U.S. Pat. No. 4,069,140 to Wunderlich discloses a method for removing arsenic or selenium from a synthetic hydrocarbon fluid by use of a contaminant-removing material. The contaminant-removing material comprises a carrier material and an active material. Carrier materials are selected from the group consisting of silica, alumina, magnesia, zirconia, thoria, zinc oxide, chromium oxide, clay, kieselguhr, fuller's earth, pumice, bauxite and combinations thereof. The active material is selected from the group consisting of iron, cobalt, nickel, at least one oxide of these metals, at least one sulfide of these metals, and combinations thereof.
U.S. Pat. No. 4,094,777 to Sugier et al. discloses a process for removing mercury from a gas or liquid. It teaches impregnation of a support only with copper and silver, although other metals can be present, for example iron. The supports taught are limited to silica, alumina, silica-alumina, silicates, aluminates and silico-aluminates; and incorporation of both metal(s) and pore-forming materials during production of the supports is taught to be necessary. Only relatively large adsorption masses are envisioned, e.g., alumina balls. Only a fixed bed reactor is taught for contacting the gas with the absorption masses, as would be appropriate for natural gas or electrolytic hydrogen decontamination, which are the only disclosed uses of the compositions and process.
U.S. Pat. No. 4,101,631 to Ambrosini et al. discloses a process for selective adsorption of mercury from a gas stream. This invention involves loading a natural or synthetic zeolite molecular sieve with elemental sulfur before the zeolite molecular sieve is contacted with the gas stream. Metal sulfides are not present in the zeolite molecular sieve when it is contacted with the gas stream. The use of pellets in adsorption beds is disclosed.
U.S. Pat. No. 4,233,274 to Allgulin discloses a method for extracting and recovering mercury from a gas. The invention requires that the gas be contacted with a solution containing mercury (II) ions and ions with the ability to form soluble complexes with such ions.
U.S. Pat. No. 4,474,896 to Chao discloses adsorbent compositions for the adsorption of mercury from hydrocarbon gas streams. Disclosed support materials are limited to carbons, activated carbons, ion-exchange resins, diatomaceous earths, metal oxides, silicates, aluminas, and aluminosilicates, with the most preferred support materials being ion-exchange resins and crystalline aluminosilicate zeolites that undergo a high level of ion-exchange. The adsorbent compositions are required to contain polysulfide species, while sulfide species may optionally also be present. Metal cations appropriate for ion-exchange or impregnation into the support material are taught to be antimony, arsenic, bismuth, cadmium, cobalt, copper, gold, indium, iron, iridium, lead, manganese, molybdenum, mercury, nickel, platinum, silver, tin, tungsten, titanium, vanadium, zinc, zirconium and mixtures thereof derived from carboxylic acids, nitrates and sulfates. The only forms of adsorbent compositions disclosed are 1/16-inch pellets.
U.S. Pat. No. 4,721,582 to Nelson discloses a composition comprising water-laden, exfoliated vermiculite that is coated with magnesium oxide for use as a toxic gas adsorbent and processes for making the same.
U.S. Pat. No. 4,814,152 to Yan discloses a composition and process for removing mercury vapor. The composition comprises a solid support that is limited to a carbonaceous support such as activated carbon and activated coke, and refractory oxides such as silicas, aluminas, aluminosilicates, e.g., zeolites. The solid support is impregnated with elemental sulfur.
U.S. Pat. No. 4,834,953 to Audeh discloses a process for removing residual mercury from treated natural gas. The process is limited to contacting the gas first with an aqueous polysulfide solution and then with a soluble cobalt salt on a non-reactive carrier material such as alumina, calcium sulfate, or silica.
U.S. Pat. No. 4,843,102 to Horton discloses a process for removal of mercury from gases with an anion exchange resin. The invention is limited in that the anion exchange resin is saturated with a polysulfide solution.
U.S. Pat. No. 4,877,515 to Audeh discloses the use of molecular sieves (zeolites) pretreated with an alkali polysulfide to remove mercury from liquefied hydrocarbons. U.S. Pat. No. 4,902,662 to Toulhoat et al. discloses processes for preparing and regenerating a copper-containing, mercury-collecting mass. The mass is made by combining a solid inorganic carrier, a polysulfide and a copper compound. Appropriate solid inorganic carriers are limited to coal, active carbon, coke, silica, silica carbide, silica gel, natural or synthetic silicates, clays, diatomaceous earths, fullers earth, kaolin, bauxite, a refractory inorganic oxide such as alumina, titanium oxide, zirconia, magnesia, silicoaluminas, silicomagnesias and silicozirconias, alumina-boron oxide mixtures, aluminates, silicoaluminates, aluminosilicate crystalline zeolites, mazzites, and cements. U.S. Pat. No. 4,911,825 to Roussel et al. discloses a process for elimination of mercury and possibly arsenic in hydrocarbons. The invention requires that a mixture of the hydrocarbon and hydrogen be contacted with a catalyst, preferably deposited on a support chosen from alumina, silicoaluminas, silica, zeolites, active carbon, clays and alumina cements, and containing at least one metal from the group consisting of iron, cobalt, nickel and palladium. Contact with the catalyst is followed by contact with a capture mass including sulfur or a metal sulfide.
U.S. Pat. No. 4,962,276 to Yan discloses a process for removing mercury from water or hydrocarbon condensate using a stripping gas. The invention is limited to the use of a polysulfide scrubbing solution for removing the mercury from the stripping gas.
U.S. Pat. No. 4,985,389 to Audeh discloses polysulfide-treated molecular sieves and the use thereof to remove mercury from liquefied hydrocarbons. The molecular sieves are limited to calcined zeolites.
U.S. Pat. No. 5,120,515 to Audeh et al. discloses a method for dehydration and removal of residual impurities from gaseous hydrocarbons. The method is limited to replacing an inert protective layer on a pellet with an active compound comprising at least one of copper hydroxide, copper oxide and copper sulfide. Materials for the pellet are limited to alumina, silicoaluminas, molecular sieves, silica gels and combinations thereof.
U.S. Pat. No. 5,245,106 to Cameron et al. discloses a method for eliminating mercury or arsenic from a fluid. The process is limited to the incorporation of a copper compound into a solid mineral support, possible calcination of the impregnated support, contact of the impregnated support with elemental sulfur and heat treatment. The solid mineral supports are limited to the group formed by carbon, activated carbon, coke, silica, silicon carbide, silica gel, synthetic or natural silicates, clays, diatomaceous earths, fullers earths, kaolin, bauxite, inorganic refractory oxides such as for example alumina, titanium oxide, zirconium, magnesium, aliminosilicates, silicomagnesia and silicozirconia, mixtures of alumina and boron oxide, the aluminates, silicoaluminates, the crystalline, synthetic or natural zeolitic aluminosilicates, mazzites and cements.
U.S. Pat. No. 5,248,488 to Yan discloses a method for removing mercury from natural gas. The method is limited to contacting the natural gas with a sorbent material such as silica, alumina, silicoalumina or activated carbon having deposited on the surfaces thereof an active form of elemental sulfur or sulfur-containing material.
U.S. Pat. No. 5,695,726 to Lerner discloses a process for removal of mercury and cadmium and their compounds from incinerator flue gas. The invention is limited to contacting a gas containing HCl with a dry alkaline material and a sorbent followed by solids separation. Activated carbon, fuller's earth, bentonite and montmorillonite clays are disclosed as sorbents having an affinity for mercuric chloride.
U.S. Pat. No. 5,846,434 to Seaman et al. discloses an in-situ groundwater remediation process. The process is limited to mobilizing metal oxide colloids with a surfactant and capturing the colloids on a phyllosilicate clay.
U.S. Pat. No. 6,719,828 to Lovell et al. teaches preparation of mercury sorbents composed of polyvalent metal sulfides precipitated from aqueous solution onto a finely divided phyllosilicate substrate in a multi-step process. The estimated manufactured cost for these sorbents is stated to be about $0.50 per pound of sorbent, compared to $0.55 per pound for activated carbon, but the sorbents are taught to be recyclable.
U.S. Pat. No. 5,653,955 to Wheelock teaches regeneration of calcium oxide used to remove hydrogen sulfide from the gases resulting from coal gasification processes. Cyclic oxidation and reduction are taught to overcome the formation of an impermeable layer of calcium sulfate on the surface of calcium sulfide particles formed by reaction of hydrogen sulfide gas with calcium oxide particles. Calcium oxide and sulfur dioxide are the products of the process taught.
No individual background art reference or combination of references teach or anticipate the compositions, processes and systems disclosed herein.